1. Field of the Invention
This invention relates to steel wire strand of the kind, hereinafter called "the kind specified", comprising a central core wire and at least one layer of wires wound around the core and the invention is concerned in particular with a method of and apparatus for making such strand.
2. Description of the Prior Art
Steel wire strand of the kind specified is normally made by laying up a plurality of wires of round, i.e. circular, cross section so as to form a strand by advancing through a closing die the individual wires from wire-carrying bobbins supported for rotation within a cage itself supported for rotation about the longitudinal axis of the closing die, the resultant strand having a cross section which is circumscribed by a circle.
In such known practice the rotation of the individual bobbins is braked merely to the extent necessary to keep the individual wires taut upstream of the closing die.
It is also known to treat the strand so produced to compact it, i.e. to deform the individual wires from their circular cross section and thus to obtain a strand with a higher fill factor than that of the uncompacted strand. (The fill factor is the percentage of the area of a circle circumscribing the section of the strand which is filled by metal). Steel wire strand is used, inter alia, for prestressing tendons in concrete structures and in some applications where space for the tendons is limited it is desired to have as much metal as possible in a given cross-sectional area so that compacted strand is then preferred to uncompacted, round-wire strand.
The present commercially used method of making compacted steel wire strand comprises drawing a length of the already formed wire strand (see U.S. Pat. No. 3,083,817) of the kind specified and which is uncompacted and in which the wires have a round cross section, through a reducing die so as simultaneously to consolidate and reduce the cross section of the strand with consequent deformation of the wires with the assistance of a specially applied back tension to the strand during the passage of the strand through the reducing die.
If one draws round-wire strand of the kind specified from a reel through a reducing die without special precautions, "bird-caging", takes place, that is to say in front of the die the outer wires lift away from the core wire and form a "birdcage". This happens because, during drawing, the outer wires have their cross-sectional areas reduced to a greater extent than the core wire. As drawing commences the outer wires go slack on the core wire upstream of the die and this slackness goes back to the reel. On the reel, however, the strand is effectively clamped due to the turns of the strand on the reel so that relative movement of the wires of the strand is not possible in the turns. As drawing proceeds, therefore, the elongation of the outer wires relative to the core wire causes bird-caging in front of the die. It is for this reason that back tension is applied to the strand, the back tension being sufficient at least plastically to elongate the core wire so that elongation of the core wire and the outer wires are the same. As normally carried out the method includes the application of a back tension sufficient to elongate not only the core wire but also the outer wires.
The back tension is normally provided by a braked capstan or drum around which the strand is advanced without slip before passing to the reducing die. Obviously the haul-off capstan to pull the strand through the die must be extremely powerful to overcome both the back tension and the drawing resistance.
The foregoing requirements for carefully controlled back tension necessitates the provision of sophisticated, powerful and relatively expensive equipment. We have now found, surprisingly, that it is possible to form compacted strand of the kind specified without applying more back tension than is normally used during the formation of uncompacted strand.
The present invention has for its object the provision of a method and apparatus for producing compacted strand of the kind specified by which it is believed that a substantial saving in plant costs and a significant saving in skilled labour costs can be effected and without any likelihood of the above mentioned tendency to bird-caging arising.